1. Field of the Invention
The present invention relates to a fuel supply device for a flexible-fuel internal combustion engine that operates on alcohol-blend fuel.
2. Description of the Related Art
Flexible-fuel internal combustion engines that operate on alcohol-blend fuel, prepared by mixing alcohol with gasoline at an appropriate ratio, have been described in the art (see Japanese Patent Application Publication No. 4-116234 (JP-A-4-116234), for example). Because alcohol contains a different number of carbon atoms from the number of carbon atoms contained in normal fuel such as gasoline, it is necessary to control the fuel injection amount in accordance with the type and concentration of the alcohol in the blended fuel in such flexible-fuel internal combustion engines. For example, ethanol has a low theoretical air-fuel ratio compared to gasoline. In the case where a blended fuel containing ethanol is used, it is necessary to increase the fuel injection amount so that the concentration of oxygen present in the exhaust is equal to that in exhaust discharged when gasoline is combusted at its theoretical air-fuel ratio. If the fuel injection amount is corrected based on the alcohol concentration in this way, a catalyst device provided in an exhaust passage is able to appropriate purify the exhaust gas. In flexible-fuel internal combustion engines, therefore, the concentration of alcohol contained in fuel is learned based on the oxygen concentration detected by an oxygen concentration sensor provided in the exhaust passage, and the fuel injection amount is corrected on the basis of the learned concentration of alcohol.
The concentration of alcohol in fuel used by a flexible-fuel internal combustion engine may vary. That is, a fuel with a different alcohol concentration from a fuel currently stored in a fuel tank may be supplied. Thus, the alcohol concentration of fuel may vary significantly when a refueling operation is performed until the alcohol concentration in a fuel supply system becomes uniform in subsequent engine operation. If the alcohol concentration of fuel stored in the fuel tank varies, still, the fuel injection amount may be corrected in accordance with the instantaneous alcohol concentration of the fuel once the alcohol concentration has been learned. In order to ensure accuracy, however, the alcohol concentration learning process cannot be started until predetermined execution conditions are satisfied. Even if the execution conditions are satisfied, and the alcohol concentration learning process is started, the alcohol concentration learning cannot be completed until the alcohol concentration in the fuel supply system becomes uniform in order to maintain the accuracy of the learned alcohol concentration. Thus, the alcohol concentration learning process must be carried out over a predetermined period time between the start and the completion of process. If the alcohol concentration of fuel is varied by a refueling operation, therefore, the fuel injection amount cannot be corrected in accordance with the alcohol concentration of the fuel until a predetermined period elapses, deteriorating the air-fuel ratio.